Digital subscriber line (DSL) technology is a technology which is frequently employed nowadays to deliver broadband services to customers. Various variations and implementations of DSL have been developed, for example ADSL, ADSL2, VDSL, VDSL2, etc. up to G.fast currently under development. All these variants will be generically referred to as DSL herein. DSL technology, during all its history, attempted to increase a bit rate so that more broadband services may be delivered to customers. Previously, wire lines like copper loops (for example from conventional telephone systems) deployed from a central office (CO) to customer premises (CPE) were employed which were rather long and did not allow transmission of data with bit rates more than a few Mb/s (megabits per second). To increase bit rates available to customers, modern access networks use street cabinets, multi-dwelling-unit (MDU) cabinets and similar arrangements which are installed close to customer premises. Such a cabinet may for example be connected to the central office by a high-speed fiber communication line, for example a gigabit passive optical network (GPON). From these cabinets, high-speed DSL systems such as Very-High-Bit-Rate DSL (VDSL2) provide connection to the customer premises.
Currently deployed VDSL2 systems (as defined e.g. in ITU-T Recommendation G.993.2) have a range of operation of about 1 km, providing bit rates in the range of tens of Mb/s. To increase the bit rate of VDSL2 systems deployed from the cabinet, recent ITU-T Recommendation G.993.5 defined vectored transmission that allows increasing upstream and downstream bit rates up to 100 Mb/s. A majority of VDSL2 systems are now deployed from cabinets and upgraded to implement vectoring operation based on G.993.5. G.fast, which is currently under development, aims at even higher bit rates and may also employ vectoring.
Power consumption is one of the key issues for cabinet deployments. Since most of DSL lines are always on, they consume power all the time, regardless whether the customer is using a service or not. In the aim to reduce the power consumption, an efficient technique of power reduction would be desirable to reduce transmit power during the time when the line is not used actively or used with reduced bit rate. For example, it would be desirable to reduce power consumption during a time the system operates with reduced bit rate (like VoiP service only) or is in sleeping mode, when only “keep alive” signals are rarely exchanged between CO and CPE.
One conventional way to reduce power is to simply switch the modem off, and customers are welcomed to do that. However, most of them do not do that, e.g. keeping the line on even at night time to avoid long waiting time for DSL startup (for vectored VDSL2 it may last up to 60-90 seconds). By the same reason it is hardly possible to save power this way in shorter breaks in data transmission during the day. Another reason is that in vectored DSL leaving (e.g. when switching off) and joining (e.g. when switching on again) of a line to a vectored group may require some adjustments in other lines, which may impact performance of existing services.
Another way is to apply a so called “low power mode” currently used in ADSL2 and also proposed for VDSL2 at some point. With ADSL low power mode, a modem monitors the incoming data traffic and turns into low transmit power and low bit loading when the required bit rate drops substantially. When the service bit rate is back at high values, the modem exits low power mode and returns to normal operation. This method is rather efficient, because the power consumption of the modem significantly depends on the value of the transmit power.
To avoid loss of data (keep the process seamless), the exit from low power mode shall be very fast; otherwise the incoming data will overflow the buffer and get lost.
One disadvantage of an L2 low power module conventionally used in ADSL is non-stationary behavior of the line. When a line going to L2, the crosstalk this line generates into other lines decreases and other lines may take an advantage of this crosstalk reduction to increase their bit rates. When the line is quickly turning back into normal operation, the crosstalk generated by this line suddenly increase, which can significantly reduce performance of other lines and even kick them out of synchronization. Thus, low power mode may cause an unstable connection.
Another problem when the modem gets back in to full power is that the tones, i.e. carrier frequencies, that were turned off during low power mode may not have a necessary minimum SNR when they come back. To avoid a communication failure, a modem in low power mode may need to monitor the line condition also for carriers that are not used. This monitoring is implemented by returning into full power mode from time to time, measure the actual SNR, and update bit loading tables to be used when modem will transition back into full power mode. This causes additional non-stationary noise which may cause unacceptable performance reduction in other operating lines.
Other conventional approaches use a predetermined set of tones that are transmitted with no change in power both in low power mode and in full power mode. However, such approaches may be problematic as regards support of vectoring, may limit power saving and/or may cause problems when the tones which are transmitted in low power mode are not usable for example due to crosstalk, narrow band interference or a notch in a loop transfer function due to a bridge tap.